Clinical evidence has shown that implantable cardioverter defibrillators (ICDs) reduce mortality, which has led to a significant increase in their use. Patients implanted with an ICD, however, potentially face the problem of defibrillator shocks delivered unnecessarily for rhythms other than ventricular fibrillation (VF) or sustained ventricular tachycardia (VT). Unnecessary ICD shocks cause undue pain, psychological disturbance and can potentially induce more serious arrhythmias in some patients. If supraventricular tachycardia (SVT), including atrial fibrillation, atrial tachycardia and sinus tachycardia, are incorrectly detected as VT, ventricular therapy may be delivered unnecessarily. Therefore, appropriately distinguishing SVT from VT may help to reduce the incidence of unnecessary shocks, without decreasing ICD efficacy.
For implanted cardiac devices, the electrical activity of the atria can provide valuable information to aid in appropriate rhythm discrimination and therapy delivery. For example, discriminators of SVT from VT can make use of intervals between P-waves associated with atrial depolarizations and R-waves associated with ventricular depolarizations. These intervals may be measured from intracardiac EGM signals as P-R intervals or R-P intervals and such intervals, along with patterns of sensed P-waves and R-waves, can be used in distinguishing between SVT and VT. The atrial electrical activity information, however, can at times be unreliable because of noise corruption, very small P-wave signal amplitude, the confounding presence of ventricular electrical activity appearing as far-field signals on an atrial signal, and dislodgement of the atrial lead.